The spray painting of automobile bodies, truck engines, appliances and other industrial goods is customarily carried out in enclosed areas called paint spray booths. These booths act to contain any fumes or oversprayed paint, to reduce the chances of airborne contamination, and to protect the painters from these hazards. The booths vary in size but are somewhat basic in their design and operation. A typical paint spray booth would consist of a work area, back section with mist eliminators and a sump.
The units to be painted generally pass through the work area while an air flow makes the oversprayed paint contact either the sump water or the spray from a water curtain. The air is scrubbed with recirculating water at a water curtain, passes through mist eliminators and is removed by an exhaust fan.
Even though paint transfer efficiencies have increased through improved application technologies, up to one-half of all paint sprayed may not reach its intended article. As a result, a significant concentration of paint builds up in the system and agglomeration can occur. The resultant mass is a sticky, tacky material which can plug mist eliminators, shower heads, and even recirculating pumps. When this happens, scrubbing efficiency decreases leading to potentially hazardous conditions of unchecked paint emissions being discharged into the atmosphere. Such conditions may also present safety hazards to paint spray booth operators.
A major problem associated with the spraying process involves the tacky or adhesive nature of the oversprayed coating materials. These tacky organic deposits are subject to bacterial growth and fungi proliferation which conditions generate corrosion and odor problems. In addition, the paint solids that are recirculated can form suspensions in the water. They remain tacky and can create expensive separation and disposal problems.
Due to environmental pressure to reduce volatile organic compounds released into the atmosphere from the use of hydrocarbon solvent (oil) based paints, manufacturers are increasingly employing waterborne paints. The use of waterborne paints creates a problem in conventional paint spray booth systems since the oversprayed paint largely remains dispersed in the water held in the sump basin. Before this water can be recirculated to the mist eliminators for removal of additional oversprayed paint, the dispersed paint previously collected must be first removed. Skimming or filtering the wastewater is ineffective since these paints exhibit no natural hydrophobicity or coagulation characteristics. Treatment programs for waterborne paints are therefore often different than treatment programs for hydrophobic oil based paints. The coagulation of dispersed paint particles is the primary objective in treatment waterborne paints. Tackiness is generally not a problem as it is with oversprayed oil based paints.
In some large industrial facilities, numerous paint spray operations may be running simultaneously. Frequently, the water which has collected the oversprayed paint may feed into a single sump. At this location the water is cleaned and then circulated back to the various spray booths from which it came. A unique problem arises at those industrial facilities in which waterborne paints are sprayed in some booths while oil based paints are sprayed in others and the water containing oversprayed paints from all booths is fed into a common sump.
Methods of detackifying water contaminated with oversprayed waterborne and/or solventborne paint are known in the art. For example, U.S. Pat. No. 5,076,939 discloses a method of detackifying paint spray operations waste by adjusting the alkalinity of the paint spray booth water and contacting the oversprayed paint with the alkalinity adjusted water and an aluminum coated silica sol, adding a flocculating agent and thereafter removing the resulting sludge. Paint spray booth water treatments for use in water based paint systems are disclosed in U.S. Pat. Nos. 5,250,189 and 5,073,205. U.S. Pat. No. 5,073,205 discloses a method for detackifying paint spray operation wastes comprising maintaining in the wash water a small amount of N-methylolacrylamide copolymer with methyldiallylamine.
While effective in treating paint spray booth water systems, metal salts and silicates do negatively impact the system being treated. Metal salts have a propensity to increase corrosion in the system and silicates can elevate foaming in the system.